Fibrinogen is a plasma protein that is involved in the coagulation of blood. It is a high molecular weight protein composed of three pairs of non-identical polypeptide chains (A alpha, B beta and gamma. The half molecules of fibrinogen are held together by symmetrical disulfide bonds in its A alpha and beta chains and in addition fibrinogen also contains interchain disulfide bridges within the half-molecule. Fibrinogen is synthesized in the liver and is secreted into the blood. In vivo studies indicate that the three fibrinogen polypeptide chains are assembled into a dimeric fibrinogen molecule at an early stage in secretion while the nascent fibrinogen is in the rough endoplasmic reticulum. In vitro studies suggest that the three fibrinogen chains are synthesized separately. These studies aim at elucidating the intracellular mechanism by which the fibrinogen chains are assembled into a functional molecule.